[AG-TECH] One-page summary of AG port usage -- please help us complete it

Colin Perkins csp at csperkins.org
Mon Feb 13 13:49:07 CST 2006

On 13 Feb 2006, at 19:15, Andrew Rowley wrote:
> It is worth noting that so long as the addresses are different for  
> each room, there should be no problem if all rooms use the same  
> port numbers.  However the problem is that with dynamic allocation,  
> you cannot be sure that you will get a different address accross  
> different venue servers.  The short-term solution to this is for  
> each person running a venue server to make sure that the multicast  
> address range is different from that of any other venue server.   
> The long-term solution is to look at some sort of venue server  
> talking where by the venue servers talk (maybe using a fixed  
> multicast address) and annouce when they are using an address, so  
> that the other servers don't use it.

That sounds similar to the old sdr model, with SAP [RFC2974], which  
started the Mbone off. Unfortunately, it's very hard to make it work  
right: to keep the bandwidth low you need a large interval between  
the announcements from each venue, but a large interval such as that  
interacts very poorly with the timers in the multicast routing  
protocol. The Cisco folks hate it, and it has caused a lot of the  
complexity and fragility in multicast routing over the years.

There have been lots of other protocols along these lines (look up  
the history of the IETF MALLOC working group, or the current MBONED  
group), but none has been successful. Not to say it's necessarily a  
bad idea, but there's a lot of history to learn from...


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